1. Field of the Invention
This invention relates to a primary coil assembly for a superconducting linear induction motor, which is suitably used for tension control or meander control of a steel strip in the manufacture of steel plates, or for controlling the stirring of molten steel in a mold or an unsolidified layer of molten steel in the central portion of uncooled steel castings of a continuous casting machine, or for controlling the amount of molten steel poured into the mold.
2. Description of the Related Art
A primary coil assembly for a superconducting linear induction motor is applied, for example, to a meander control of a steel strip in the manufacture of steel plates, as shown in FIG. 1. A pair of primary coil assemblies 11 of an induction motor are arranged on each side of a traveling rolled or half-rolled strip 10, and the paired motors are positioned above and below the strip 10, respectively, at a predetermined distance therefrom. An alternating current is always supplied to the primary coil assemblies 11, such that when the traveling strip 10 meanders in a direction at right angles to the traveling direction, i.e., in the width direction of the strip, at the location of the primary coil assemblies, a thrust acts on the strip 10 in a direction opposite to the direction of such displacement, to thereby hold the strip 10 in position at all times.
In the primary coil assembly for a superconducting linear induction motor, saddle coils shown in FIGS. 2 to 4 are conventionally used. In the case of a three-phase alternating-current motor, the coil assembly comprises three saddle coils 12, 13 and 14 for U, V and W phases, superposed one upon another, and is positioned at a predetermined distance x from the strip 10 serving as a secondary conductor, as shown in FIG. 4. The phase pattern of currents supplied to the coils of this induction motor is U, -V, W, -U, V, -W.
Each coil is impregnated with epoxy resin and unified, to prevent quenching from occurring due to friction heat produced by an accidental movement of the winding. Further, since a superconducting wire constantly produces heat due to alternating current loss, cooling channels are provided between the windings of the coils for circulating liquid helium therethrough.
As described above, the conventional primary coil assembly for a three-phase superconducting linear induction motor includes three saddle coils stacked one upon another and requires space for the cooling channels, and therefore, these three coils are different in size and in the number of turns, and hence have different impedances. Consequently, the induction motor including the saddle coils is unable to produce a thrust uniformly from the sequentially arranged coils.
Furthermore, since the aforementioned primary coil assembly for an induction motor has the shape of a saddle, it is difficult to wind a superconducting wire with a uniform tensile force applied thereto and increase the tensile force itself. Therefore, although the windings are each impregnated with epoxy resin to be unified, they are liable to be moved, and thus, quenching is likely to occur due to friction heat produced by the movement of the windings. Moreover, in the aforementioned arrangement of stacked saddle coils, the phase pattern of coil currents is limited substantially to U, -V, W, -U, V, -W.